Metal halide discharge lamps usually include discharge vessels made of high silicon content glass, and especially of quartz glass, see for example U.S. Pat. No. 4,689,519. Quartz glass is corrosion resistant with respect to vaporized metal halides. Difficulties with quartz glass, however, are unfortunately endemic, for example due to the possibility of devitrification, and insufficient acceptance of thermal loading. In general illumination use, the maximum temperature that quartz glass can be used with is about 1000.degree. C. The poor heat conductivity of quartz glass, namely about 0.1 W/mK, also interferes with the illumination qualities of the lamp which, on the other hand, are important in general service illumination.
It has been tried to construct metal halide discharge lamps with ceramic discharge vessels. U.S. Pat. No. 4,687,969 describes such an experiment, which utilizes a ceramic discharge vessel made of aluminum oxide, Al.sub.2 O.sub.3. The thermal loading of the vessel can be substantially improved, since the maximum temperature of the material for lamp use is about 1700.degree. C. On the other hand, however, problems in connection with resistance to corrosion arise, particularly in the region of the seal between the ceramic vessel and an end plug. The use of a niobium tube, well known in the construction of sodium high-pressure discharge lamps, which has a thermal coefficient of expansion roughly matched to that of aluminum oxide, cannot be used due to the lack of corrosion resistance. It has, therefore, been proposed, and as described in U.S. Pat. No. 4,687,989, to use Cermet plugs. One of the Cermet plugs is sintered into the tube, and the other end of the tube has the Cermet plug sealed in by means of a frit. This solution is expensive to manufacture and the lifetime of the lamp is not satisfactory.
Aluminum nitride, AlN, has been used as a ceramic substrate in the semiconductor technology field see U.S. Pat. No. 4,766,097. This material, as used for semiconductor substrates, is not transparent.